Intramedullary Medical Devices and Methods of Manufacture

ABSTRACT

Intramedullary medical devices (e.g., intramedullary nails) and methods for their use and manufacture are described herein. The intramedullary medical devices described herein may provide sustained compressive forces across a bone fusion site despite bone resorption processes. Through various embodiments, the intramedullary medical devices described herein may provide non-linear force curves relative to displacement. Intramedullary medical devices are described with multiple elements made of different materials. Examples of intramedullary medical devices are described with shape memory alloys.

BACKGROUND

Tibio-talo-calcaneal (TTC) ankle fusion is a technique which may be usedin order to achieve functional, stable, and pain-free orthopedic fusionfor the treatment of appropriate medical conditions. Intentional bonefusions which are unsuccessful can lead to patient pain, recurringsurgery, infection, loss of limb function, and/or, in extreme cases,limb amputation.

SUMMARY

Intramedullary medical devices (e.g., intramedullary nails) and methodsfor their use and manufacture are described herein. The intramedullarymedical devices described herein may provide sustained compressiveforces across a bone fusion site despite bone resorption processes.Through various embodiments, the intramedullary medical devicesdescribed herein may provide non-linear force curves relative todisplacement. Intramedullary medical devices are described with multipleelements made of different materials. Examples of intramedullary medicaldevices are described with shape memory alloys.

In one aspect the disclosure describes an intramedullary medical device,including a first element with a first anchor hole adapted to accept afirst bone anchor, a second element with a second anchor hole adapted toaccept a second bone anchor, and a third element which is compliant togreater than about 1 percent strain without plastic deformation, whereinthe third element is connected to the first element and connected to thesecond element.

In another aspect, the disclosure describes an intramedullary medicaldevice, including a sleeve with a closed end, an insertion tip connectedto the closed end, wherein the insertion tip includes a firstthrough-hole adapted to receive a first bone anchor. The intramedullarymedical device also includes a shape memory alloy element attached to afirst interior surface of the sleeve. The intramedullary medical devicealso includes an anchor element attached to the shape memory alloyelement, wherein the anchor element includes a second through-holeadapted to receive a second bone anchor, wherein the anchor element andthe sleeve are slidably interconnected.

In another aspect, the disclosure describes an intramedullary medicaldevice, including a proximal anchor element adapted to be fixed to ahuman tibia, a shape memory alloy element connected with the proximalanchor element, and a distal anchor element connected with shape memoryalloy element, wherein the distal anchor element is adapted to be fixedto a human calcaneus, wherein the distal anchor element is slidablyinterconnected with the proximal anchor element.

These and various other features as well as advantages will be apparentfrom a reading of the following detailed description and a review of theassociated drawings. Additional features are set forth in thedescription that follows and, in part, will be apparent from thedescription, or may be learned by practice of the described embodiments.The benefits and features will be realized and attained by the structureparticularly pointed out in the written description and claims hereof aswell as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

A BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example use of an intramedullary device used in a TTCprocedure.

FIG. 2A shows an embodiment of an intramedullary medical device.

FIG. 2B shows an embodiment of a proximal anchor element attached to arigid element of an intramedullary medical device.

FIG. 2C shows an embodiment of a contracting element of anintramedullary medical device.

FIG. 2D shows a stress-strain curve of an example of a shape memoryalloy exhibiting pseudo-elastic properties while loading and unloadingstrain.

FIG. 2E shows an embodiment of a distal anchor element.

FIG. 3A shows an embodiment of a locking system including a key sectionand a channel which is stepped to provide a locking function.

FIG. 3B shows an embodiment of a locking system including a key sectionand a channel with the key section rotationally locked within thechannel.

FIG. 4 shows an embodiment of an intramedullary medical device with analternative embodiment of a rigid element.

FIG. 5 shows an alternative embodiment of a contracting element for anintramedullary medical device.

FIG. 6 shows an embodiment of an intramedullary medical device with adistal anchor element which extends outside the rigid element.

DETAILED DESCRIPTION

The following description of various embodiments is merely exemplary innature and is in no way intended to limit the disclosure. While variousembodiments have been described for purposes of this specification,various changes and modifications may be made which will readily suggestthemselves to those skilled in the art, and which are encompassed in thedisclosure.

Unless otherwise indicated, all numbers expressing quantities,measurements (e.g., strains, stresses), properties, and so forth used inthe specification and claims are to be understood as being modified inall instances by the term “about.” Accordingly, unless indicated to thecontrary, the numerical parameters set forth in the followingspecification and attached claims are approximations that may varydepending upon the desired properties sought to be obtained. At the veryleast, and not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, will inherently contain certainerrors necessarily resulting from the standard deviation found in itstesting measurements.

FIG. 1 shows an example use of an intramedullary device used in a TTCprocedure. TTC is a procedure which allows bones in the ankle of apatient to fuse over time. The tibia 102, talus 104, and calcaneus 106are held against each other and boney in-growth between the bones isfacilitated. The boney in-growth is facilitated through anintramedullary medical device 108. An intramedullary medical device 108is positioned inside a bore created in a patient's tibia 102 (roughlyalong the long axis A of the tibia) and anchored proximally via a boneanchor such as a bone screw 110. The procedure may create substantiallyparallel bores in the patient's talus 104 and calcaneus 106, allowingthe intramedullary medical device to pass through the two bones on theway to the proximal anchor site in the patient's tibia 102. Additionalbone anchors (e.g., bone screws 110) may be placed in the patientscalcaneus 106, thereby allowing the intramedullary device to hold thepatient's tibia 102, talus 104, and calcaneus 106 under compressionwhile the bones fuse over time. A bone anchor(s) may be placed in thepatient's tibia before or after a bone anchor(s) is placed in thepatient's calcaneus.

In the embodiment shown, bone anchors are placed in a medial-lateraldirection in the patient's tibia and calcaneus. Bone anchors may beplaced in an anterior-posterior direction, or on an angle to amedial-lateral direction and/or anterior-posterior direction. Theillustration herein of bone anchors placed in a medial-lateral directionis not meant to limit the disclosure or any claim to a particularplacement direction of bone anchors.

Boney in-growth is achieved over time in a TTC procedure and the processof boney in-growth may result in the interfaces between bonescompressing (e.g., as old bone compacts/resorbs, as new bone is formed)thereby resulting in a shortening of the distance between bone anchors.The compressing of bone interfaces and/or shortening of distance betweenbone anchors may result in a loss of the compressive stress (e.g.,“compression”) between bones provided by the intramedullary medicaldevice 108. A loss of compression between the bones may result in anunsuccessful or prolonged fusion time for the bones and should otherwisebe avoided. The intramedullary medical device 108 described hereinlimits the loss of compressive stress across bone interfaces due to ashortening of distance between bone anchors and thereby providesimproved opportunities for bone fusion while boney in-growth occurs.

Intramedullary medical devices may be used in applications other thanTTC procedures. For example, bones in the hand or foot may be fusedusing an intramedullary medical device as described herein, but whichhas been sized and/or shaped appropriately for the smaller bones. Otherbones which form joints may also be fused through suitable sizing andshaping of an intramedullary medical device described herein. As anotherexample, vertebrae in the spine may be fused.

Fractured bones may be held or set using an intramedullary medicaldevice. For example, a fracture in a long bone (e.g., tibia, femur,humerus, ulna) may be held or set using an intramedullary medicaldevice, as described herein. Fractures in other bones may also be heldor set. An intramedullary medical device that is used to hold or set abone fracture may be sized and/or shaped based on the size and/or shapeof the fractured bone and/or intramedullary implantation procedure usedfor the fractured bone. Different processes of bone growth may befacilitated through different levels of stress applied to the bonesand/or fractured portions of bone. For example, bone fractures may befused through the use compressive stresses that are different from thestresses used for fusing bones in a joint.

Compressive stress is sustained in embodiments described herein throughthe use of non-linear compressive or expansive elements. Examples ofnon-linear compressive or expansive elements include non-linear springsand materials with non-linear stress-strain behavior. For example, ashape memory alloy (SMA) material may be used to provide a non-linearstress-strain relationship. As another example, an elastomer materialmay be used in a compressive or expansive element.

In one embodiment, non-linear springs or other mechanical configurationsmay be utilized for non-linear stress/strain behaviors in compressive orexpansive elements. A spring may be a non-linear spring, a linearspring, a constant-force spring, or another type of spring. The materialof the spring may be a metal or other suitable material. The spring mayhave shape memory properties, and/or pseudo-elastic properties. Thespring may be contained in a sleeve or other protective covering tolimit the possibility of unfavorable interaction between the spring andrest of the intramedullary device while stretching and/or contracting ofthe spring occurs. For example, the spring may be encased to avoidinteractions between the spring while the spring is loaded with anexpansive strain and/or while the spring recovers through contraction.

Another embodiment described herein of an intramedullary medical device108 includes an SMA used as a contracting element between proximal anddistal anchor sites of a patient. The SMA may be strained past anon-linear transition in effective modulus (further described below),such that the SMA exhibits pseudo-elastic properties, including a lowereffective modulus. Depending on the treatment of the SMA, the decreasein modulus after the nonlinear transition may be significant. Relativeto the total force applied to the SMA to strain it past the nonlineartransition (e.g., at about one percent strain), the increase in forcerequired to strain the SMA through to six percent strain or more may berelatively much less. Approximately, some SMAs may appear to requireonly a relatively constant force to be strained from the nonlineartransition to a greater strain (e.g., near a plastic deformation point).

An SMA in this strain region can exhibit pseudo-elastic properties (alsoreferred to as “super-elastic” properties). While in this strain region,the SMA also may recover strain without showing a significant decreasein force. In other words, in an intramedullary medical device using anSMA contracting element, the SMA may recover strain with little or nodecrease in applied force across the junction between the bones.Pseudo-elastic behavior of SMAs is explained further with respect toFIG. 2D.

In a TTC procedure, the proximal anchor element is anchored to theproximal anchor site in the patient's tibia. As one example, the distalanchor element is fixed to the distal anchor site (e.g., the patient'scalcaneus) after the proximal anchor element has been fixed to thepatient's tibia. As another example, the distal anchor element is fixedto the distal anchor site before the proximal anchor element has beenfixed to the patient's tibia. The contracting element (between theproximal anchor element and the distal anchor element) holds the bonesunder compression while the boney in-growth and fusion process occurs.The term fixed, as used herein with relation to an element of anintramedullary medical device being fixed to a bone, refers to theattachment of the element of the intramedullary medical device beingsubstantially attached to the bone through the use of a bone anchor orother means.

Strain may be induced into a contracting element through stretching thecontracting element before both the distal anchor element and theproximal anchor element are fixed to the patient's bone(s). In theexample of a TTC procedure, the recovery of the strain in thecontracting element may occur while boney in-growth occurs between thepatient's tibia, talus, and calcaneus. The contracting element providescompression between the tibia, talus, and calcaneus while the boneyin-growth occurs, despite the decrease in distance between the proximaland distal anchor sites. The details of the recovery of strain in thecontracting element are described further below. The compression whichis maintained between the tibia, talus, and calcaneus increases thechances of a successful fusion between the bones.

There may be torsional and/or bending forces on the intramedullarymedical device. The forces may evolve from many sources, includingforces involved with the installation of the medical device, interactionbetween the bones while under compression, forces from the patient'smovement (e.g., flexing muscles), the boney in-growth process, outsideforces, stored torsional strains and/or bending strains. As illustratedby the embodiments disclosed herein, the intramedullary medical devicemay include features that limit the effects of these torsional and/orbending forces on the process of boney in-growth and the successfulfusion of the patient's tibia, talus, and calcaneus.

As used herein with respect to the intramedullary medical device orelements thereof, certain reference terms are used. As used herein, theterm “axial” refers to a vector along the long axis A of the device (asshown in FIG. 1) or an axis parallel to the axis A. As used herein, theterm “radial plane” refers to a plane perpendicular to axis A, such as aplane P. As used herein, the term “torsion” refers to rotation within aradial plane. As used herein, the term “radial” refers to a vectorperpendicular to and with reference to an axis A or an axis parallel toaxis A. For example, a radial vector may be a vector in a particularradial plane. As used herein, the terms “bend” or “bending” refer todistortion of an element of the medical device out of alignment withaxis A. For example, a portion of the medical device may be aligned withaxis A before being bent, and may be aligned at an angle to axis A afterbeing bent.

As used herein, the term “strain” (used without a qualifier) is used torefer to engineering strain, or the local axial distortion of a materialdivided by the length of that material along the axis of distortion.Strains as referred to herein are therefore dimensionless. The term“absolute strain” is used herein to refer to distortion expressed inunits of length. Strains refer to local distortion of a material, and donot refer to a distortion between points of an element that is achievedby a mechanical arrangement (e.g., a spring). For example, a coil springmay be extended through twisting of the coil of the spring such that thetotal length of the coil spring is extended 10 percent (10%) or more,yet no part of the coil is strained 10 percent (10%).

Various embodiments of intramedullary devices are described below whichhave properties of axial compliance and rigidity in torsion and bending.The axial compliance of the intramedullary device allows strains, whichare imparted before and/or during surgery, to be recovered in mannersthat aid bone fusion within the patient. The torsional and bendingrigidity both promote bone fusion within the patient by limitingmovement across the bone fusion sites.

FIG. 2A shows an embodiment of an intramedullary medical device 200. Theembodiment shown includes a proximal anchor element 202, a distal anchorelement 204, a contracting element 206 which connects the anchors, and arigid element 208 between the anchors which slidably couples theanchors. The embodiment shows the elements as assembled in theintramedullary medical device 200. Each element is discussed furtherbelow.

The rigid element 208 may slidably connect the proximal anchor element202 and the distal anchor element 204. In the embodiment shown, therigid element 208 is a tube-like structure that provides resistance toboth torsional and bending stresses. The rigid element 208 is shownenclosing the distal anchor element 204, limiting movement of the distalanchor element to axial movement with respect to the rigid element. Forexample, the rigid element 208 and the distal anchor element 204 may bemated and/or slidably coupled, as described further below, such that thedistal anchor element may move axially with respect to the rigidelement, but may not move torsionally and/or bend (e.g., deform out ofaxis) with respect to the rigid element.

In the embodiment shown, the proximal anchor element 202 is connected tothe rigid element 208. The proximal anchor element 202 may be attachedto the rigid element 208, or the proximal anchor element may be formedwith the rigid element as a single piece of material. In other words,the proximal anchor element 202 and the rigid element 208 may beportions of a single element. For example, the proximal anchor element202 and the rigid element 208 may be portions of a single element. Inanother embodiment, the proximal anchor element 202 may be separatedfrom the rigid element 208 or connected with the rigid element through aflexible interconnect.

A contracting element 206 is shown linking the distal anchor element 204and the proximal anchor element 202. The contracting element 206 may bestrained (e.g. stretched toward open end 210 of the rigid element)through sliding the distal anchor element 204. Thereby, the distalanchor element 204 may slide relative to the proximal anchor element 202and the rigid element 208. In one embodiment, the contracting element206 and the distal anchor element 204 are elements which may beconnected. In another embodiment, the contracting element 206 and thedistal anchor element 204 are both manufactured from a single piece ofmaterial.

The rigid element 208 has slots 211 disposed allowing the anchor holes212 of the distal anchor element 204 to be accessed while the distalanchor element is slid relative to the rigid element. For example, theslots 211 may allow a bone anchor(e.g., bone screw, bone pin, rod) topass through the rigid element 208 without causing interactions betweenthe bone anchor and the rigid element that would axially limit thesliding of the distal anchor element 204 relative to the rigid element.The slots 211 may be designed to guide axial travel of the bone anchorwhile it is coupled with the distal anchor element. In the embodimentshown, the slots 211 are shown large enough for the distal anchorelement 204 to be slid axially. For example, the slot 211 may guide thebone interface device to travel axially, while limiting torsionalmovement of the bone interface device.

The proximal anchor element 202 and distal anchor element 204 maycomprise a material selected for interfacing with a bone anchor. Thematerial of the proximal anchor element 202 may be different than thematerial of the rigid element 208. The material of the distal anchorelement 204 may be different from the material of the contractingelement 206. Each material may be selected for different properties(e.g., properties adapted for holding a screw rather than properties forcontracting and/or properties for compliance along an axis).

FIG. 2B shows an embodiment of a proximal anchor element 202 attached toa rigid element 208 of an intramedullary medical device. In theembodiment shown, the rigid element 208 is shown generally as a tube byway of example. Other embodiments may be adapted and used wherein therigid element 208 is a sleeve of arbitrary cross-section. For example,the rigid element 208 may be a sleeve with a rectangular cross section,or a star-patterned cross-section. As further examples, the sleeve mayhave a cross section that may include ridges, channels, solid regionsand/or voids. The cross section of the sleeve may be adapted to achievecertain torsional resistance and/or bending resistance characteristics.In other embodiments, the sleeve structure may be replaced or buttressedby a rib, strut, and/or crutch structure. Further embodiments of rigidelements are described further below.

In the embodiment shown, the proximal anchor element 202 has acylindrical shape with radial anchor holes 212 (e.g., perpendicular tothe cylinder). In some embodiments, the end 213 of the proximal anchorelement is pointed to ease insertion into a bone cavity that has beenprepared in the patient's tibia. The end of the proximal anchor element202 may be shaped in order to ease insertion, to optimize structuralstrength of the proximal anchor element, and/or to benefit interactionbetween the intramedullary device and the patient.

In the embodiment shown, the proximal anchor element 202 is attached tothe rigid element 208 to form a single element 220. In anotherembodiment, the proximal anchor element 202 is formed together with therigid element 208 to form a single element 220. The single element 220as shown includes a closed end formed by the proximal anchor element202, and an open end 210, as further described herein. Also shown anddescribed further herein are slots 211 in the rigid element 208.

FIG. 2C shows an embodiment of a contracting element 206 of anintramedullary medical device. As described further herein, thecontracting element 206 may comprise an SMA. In the embodiment shown,the contracting element 206 includes a rod of SMA. In another embodimentthe contracting element 206 includes a plurality of rods of SMA. As usedherein, the term “rod” is used to describe an elongate element, whichmay be cylindrical and may be solid. In yet another embodiment,described further below, the contracting element 206 is a tube of shapememory alloy with slots and/or voids placed in the tube. In yet anotherembodiment, the contracting element 206 includes a combination of rodsand tubes. Other embodiments of contracting elements 206 are describedherein, including other structures and/or other materials, such aslinear springs, non-linear springs, and composite structures.

As further described herein, a contracting element 206 provides axialcompliance between a proximal anchor element and a distal anchor elementof an intramedullary medical device. As used herein, “axial compliance”refers to the capability of achieving greater than about one percent(1%) strain without plastic deformation. As used herein, “plasticdeformation” refers to strain that is unrecoverable (or not otherwiserecoverable under anticipated in-vivo conditions) in the absence ofstress.

The SMA may be used as a contracting element 206 through straining theSMA into a pseudo-elastic state. The SMA may be treated to enhance thepseudo-elastic properties of the alloy. As described further herein withrespect to FIG. 2D, the pseudo-elastic properties of an SMA may bedetermined to start occurring at a nonlinear transition in astress-strain curve of the shape memory alloy. In practice, thisnonlinear transition may occur between about one percent (1%) and aboutsix percent (6%) axial strain. Other nonlinear transition regions may bedeveloped through, for example, treatments to the SMA before, during,and/or after straining.

In the embodiment shown, the contracting element 206 has a first thread214 on its proximal portion, which interfaces with the proximal anchorelement and/or a proximal portion of the rigid element when theintramedullary device is assembled. In the embodiment shown, thecontracting element 206 has a second thread 215 on its distal end, whichinterfaces with the distal anchor element when the intramedullary deviceis assembled. In an embodiment, the first and second threads 214, 215may be threads with an opposite handedness. For example, the firstthread 214 may be right-handed and the second thread 215 may beleft-handed. As another example, the first thread 214 may be left-handedand the second thread 215 may be right-handed.

In one embodiment, a right-handed thread 214 is disposed on the proximalportion 219 of the contracting element 206 and the right-handed thread214 interfaces with a right-handed thread on the proximal anchor elementand/or a proximal portion of the rigid element.

The threads on the contracting element 206 may be disposed externallyand/or internally. In the embodiment shown, the contracting element 206has an external thread 214 (e.g., on the exterior of the contractingelement), which engages the proximal anchor element and/or the rigidelement when the intramedullary medical device is assembled. In anotherembodiment, the contracting element 206 may have an internal thread(e.g., on the interior of the contracting element) which engages theproximal anchor element and/or the rigid element. In another embodiment,the contracting element 206 may have both external and internal threads.For example, a contracting element 206 may have a pair of threads, oneexternal and one internal, on a proximal portion 219 of the contractingelement and the internal thread may interface with the proximal anchorelement while the external thread may interface with the rigid element.

In the embodiment shown, the contracting element 206 also has anexternal left-handed thread 215 disposed on the distal portion 217 ofthe contracting element and that thread interfaces with a left-handedthread on the distal anchor element. A tool interface (e.g., hex socket,screw head) may be disposed on the distal portion 217 of the contractingelement 206, allowing the contracting element to be turned. In anotherembodiment, the threads 214, 215 are disposed as described above, butthe handedness of the threads is reversed.

In an embodiment, the intramedullary medical device may be assembled byplacing a proximal portion 219 of the contracting element 206 in contactwith a proximal anchor element and/or the rigid element such thatrespective starting portions of the threads (e.g., thread 214 and athread on an interior surface of the proximal anchor element) areadjacent to each other. The assembly may continue with the placement ofthe distal anchor element in contact with the distal portion of thecontracting element such that respective starting portions of thethreads (e.g., thread 215 and a thread on the distal anchor element) areadjacent to each other. A locking system, as described further herein(e.g., a system of mating ridges and channels), may be engaged betweenthe distal anchor element and the rigid element.

With the proximal thread 214 and distal thread 215 of the contractingelement 206 adjacent to the respective starting portions of the threadson the other elements of the intramedullary medical device, thecontracting element 206 may be turned in a clockwise direction in orderto engage the contracting element with the proximal anchor element(and/or rigid element), while the same rotation will also engagecontracting element with the distal anchor element. The turning of thecontracting element 206, with the respective threads engaged, drives thedistal anchor element closer to the proximal anchor element. Otherembodiments of intramedullary medical devices, with different lockingsystems between elements, as described further herein, may also beadaptable to similar rotational assembly techniques.

Embodiments may be designed such that two threads disposed on different(e.g., proximal and distal) portions of the contracting element 206 neednot be different handedness. For example, the proximal and distalthreads 214, 215 on the contracting element 206 may be right-handed orleft-handed. As another example, two threads (e.g., proximal internalthread and proximal external thread 214) may be right-handed and anotherthread (e.g., distal thread 215) may be left-handed.

In another embodiment, the contracting element 206 may have otherlocking features that engage with the other elements of theintramedullary medical device. For example, the contracting element 206and other elements of the intramedullary medical device may interfacethrough a locking system of cams, posts, extensions, slots, channelsand/or guides. A locking system can provide the ability for one of theelements to rotate with respect to another element during assembly, andmay resist the opposite rotation (e.g., including disassembly) in anopposite direction after such assembly. It should be understood that theabove discussion of threads is another example of a locking system whichallows rotation in one direction during assembly, and resists counterrotation (and hence, disassembly) after such assembly. Therefore, theabove discussions of threads, handedness, and assembly techniques may beapplied to other locking systems, including rotational locking systems,as appropriate.

In other embodiments, an expanding element is substituted for thecontracting element 206. For example, the expanding element may beconfigured within the rigid element to cause the proximal anchor elementand distal anchor element to compress the bones of the tibia, talus, andcalcaneus. For example, the expanding element can “push” the distalanchor element axially with respect to the rigid element via a suitableinterior configuration of the expanding element, as well as the proximalanchor element, the rigid element, and the distal anchor element.

FIG. 2D shows a stress-strain curve of an example of a shape memoryalloy exhibiting pseudo-elastic properties while loading and unloadingstrain. The curve shown includes regions where the SMA displaysdifferent effective moduli. Through an initial portion of the curve 220(the initial loading of strain on the SMA), the effective modulus of theSMA is larger than in the pseudo-elastic region of the curve 221. Afterthe SMA is strained past a pseudo-elastic transition region 223, the SMAexhibits a smaller effective modulus. Depending on the properties of thepseudo-elastic region 221 of the SMA's stress-strain behavior, theeffective modulus may be significantly less than the modulus of theinitial straining of the SMA. In other words, the force exhibitedresisting strain recovery and/or further straining of the SMA may berelatively constant with respect to incremental change in strain of theSMA.

The stress-strain curve shows hysteresis 222 between loading of strainon the SMA and the release of strain by the SMA, as indicated by thearrows on the curves 224, 226. In the embodiment shown, the effectivemoduli of the loading curve 224 and the unloading curve 226 in thepseudo-elastic region 221 of the curve are similar. However, thesemoduli may be different from each other in other embodiments. Theunloading curve 226 shows a decrease in stress relative to the loadingcurve 224 through the unloading path of releasing strain. There is asignificant drop in stress (indicated as hysteresis 222) exhibited bythe SMA at the onset of unloading followed by a relatively constantstress section of the unloading curve 226.

An SMA with a non-linear stress-strain behavior may be used to providerelative maintenance of stress exhibited by the contracting elementthrough a range of unloaded strain (e.g., along the unloading curve226). For example, operation through the unloading curve 226 of thepseudo-elastic region 221 of the SMA allows the contracting element tounload strain (e.g., shorten the distance) between the proximal anddistal anchor elements while maintaining a relatively constant stressbetween the two elements.

In some embodiments, an element of SMA may be designed and/or adapted tobe in the pseudo-elastic region 221 for strains, which are utilized inthe fusing of bones. A TTC procedure allows bones to fuse over time, andthere may be an expected amount of travel between the bone anchors dueto the bone fusion process. A contracting element made of SMA may bedesigned such that the expected amount of travel may be smaller than thepseudo-elastic region 221, in terms of absolute strain of thecontracting element. For example, a contracting element may exhibitpseudo-elastic behavior throughout a bone fusion process if thecontracting element's pseudo-elastic region 221 is exhibited over alarger absolute strain than the expected amount of travel. Some examplesof absolute strain ranges corresponding to pseudo-elastic regions ofcontracting elements for TTC procedures may include 0 to 15 millimeters,0 to 10 millimeters, 0 to 8 millimeters, 0 to 6 millimeters, 0 to 5millimeters, 1 to 15 millimeters, 1 to 10 millimeters, 1 to 8millimeters, 1 to 6 millimeters, and 1 to 5 millimeters.

Stress-strain curves similar to the curve shown in FIG. 2D may bedeveloped for compressive strains on an SMA. The magnitude of thecompressive stresses and compressive strains may differ from the tensionstresses and tension strains shown in FIG. 2D. As described furtherherein, configurations of the proximal and distal anchor elements of theintramedullary medical device may provide compression between thoseelements through the stored compressive strain in a dynamic elementwhich expands (e.g., an expansive element). In other words, recovery ofthe compressive strain through expansion may be configured to providecompressive forces between the proximal anchor element and the distalanchor element. As further described herein, the sustained compressiveforces between the proximal anchor element and the distal anchor elementprovide medical advantages when used with patients (e.g., in a bonefusion application).

FIG. 2E shows an embodiment of a distal anchor element 204. The distalanchor element 204 includes at least one anchor hole 212. In thisembodiment, the distal anchor element 204 is adapted to fit within therigid element, as described further herein. The distal anchor element204 may contact the rigid element along some or all of outer surface ofthe distal anchor element. In the embodiment shown, the distal anchorelement 204 contacts the rigid element along substantially all of theouter rounded surface 234 of the distal anchor element but not the endsurfaces 236. In another embodiment, the distal anchor element 204extends beyond the rigid element. For example, other embodiments arefurther described below in which a significant portion of the distalanchor element 206 and/or a peripheral element is/are outside the rigidelement.

Guiding and/or locking elements 238 may be included on a distal anchorelement 204 which can interface with a rigid element to limit torsionalmovement of the distal anchor element with respect to the rigid element.The guiding/locking elements 238 may interface the distal anchor element204 and the rigid element through a mating of ridges and channels. Forexample, the distal anchor element may have a ridge 238 which mates witha channel in the rigid element, thereby allowing axial movement andlimiting torsional movement of the distal anchor element 204. As anotherexample, the distal anchor element 204 may have a channel which mateswith a ridge in the tube, thereby allowing axial movement and limitingtorsional movement.

The ridges 238 of the distal anchor element 204 may slidablyinterconnect with the rigid element. The ridges 238 may allow relativemovement axially (e.g., along axis A) between the rigid element and thedistal anchor element 204. The ridges 238 may prevent torsional movementbetween the rigid element and the distal anchor element 204.

The embodiment shown includes interior threads 232 which mate to thecontracting element during assembly of the intramedullary medicaldevice. Access to the distal end of the contracting element may beprovided through end surface 236 and through cavity 230 at the distalend of the distal anchor element 204. Thereby, the contracting elementmay be accessed in order to allow rotation (or other locking) of thecontracting element to facilitate assembly, as described further herein.

Sizing or shaping of the intramedullary medical devices described hereinmay include bending or curving one or more elements of the device. Forexample, rigid elements described herein may be adapted to particularapplications through being curved (e.g., fusing a joint in a curvedposition).

Elements of an intramedullary medical device as described herein may berearranged and/or modified. For example, an anchor element may beexpanded or reduced in size as appropriate for a particular bone. Asanother example, a curvature may be introduced into an element, such asa rigid element. A compressive element may be placed in a straight orcurved portion of an intramedullary medical device. Interconnections ofthe elements as described further herein (e.g., slidableinterconnections, threads) may be positioned according to anyrearrangement, and/or modification of the elements.

FIG. 3A shows an embodiment of a locking system 300 including a keysection 302 and a channel 304 which is stepped to provide a lockingfunction. For example, the locking system 300 may be used to easeassembly, to assure that correct assembly has occurred, and/or to limitdisassembly (e.g., through inadvertence, through mishandling).

In one embodiment, the locking system 300 is disposed between distalanchor element and the rigid element of the intramedullary medicaldevice. As an example, a ridge on a distal anchor element may have aportion (e.g., a key section) which mates (e.g., couples, fits,interfaces with) with a portion of a channel in a rigid element. Inanother embodiment, as described further below with respect to FIG. 6,the locking system 300 may be disposed between the rigid element and aninterface element.

During assembly of the intramedullary medical device, the dynamicelement (e.g., contracting element, expanding element) may be connectedafter the locking system 300 is engaged. Thereby, the locking system 300may maintain alignment of elements of the intramedullary medical deviceduring, for example, assembly (including installation of the dynamicelement), shipment, storage, insertion of the device in the patient,and/or straining of the dynamic element.

In the embodiment shown, the ridge includes a key section 302 whichmates with the channel 304 through axial sliding and rotation.Initially, the key section 302 may be slid axially as shown by the arrow306. In the embodiment shown, after the key section 302 meets a proximalaxial stop 310 in the channel (so named because it limits proximalmovement), the key section may be rotated 312. In other embodiments, theproximal axial stop 310 need not be reached before the rotation 312 canoccur. In the embodiment shown, a distal axial stop 314 must be passedby a distal portion 316 of the key section 302 before rotation 312 canoccur.

The embodiment shown limits rotational movement 312 of the distal anchorelement (which includes the ridge and key section 302) relative to therigid element (which includes the channel 304) while the ridge andchannel are interfacing both before and after the ridge and channel havebeen rotationally mated. This limitation in relative rotational movementis accomplished through interfacing of the upper extents of the keysection 311 with the upper extents of the channel 313 and/or throughinterfacing of the lower extents of the key section 317 with the lowerextents of the channel 315. The interfacings of the extents of the ridgeand channel 304 may occur both before and after the ridge and channelare rotationally mated.

FIG. 3B shows an embodiment of a locking system 300 including a keysection 302 and a channel 304 with the key section rotationally lockedwithin the channel. With the key section 302 and the channel 304 in alocked rotational position, as shown, the key section is limited frommoving axially in a distal direction past the distal axial stop 314 withrespect to the channel unless the key section 302 is counter-rotated 318with respect to the channel 304. For example, the distal axial stop 314may prevent inadvertent or unintended disassembly of the distal anchorelement from the rigid element.

In the embodiment shown, at the point where the key section 302 isbetween the proximal axial stop 310 and the distal axial stop 314,movement is limited to rotational movement 312, 318. Rotational movement312, 318 may occur for a determined angular rotation, as determined bythe channel 304.

In the embodiment shown, during rotation the distal axial stop 314limits axial movement of the key section 302 through interference withthe distal portion 316 of the key section. Also, during rotation 312 theproximal axial stop 310 prevents further proximal axial movement untilthe rotation 312 is completed. After rotation 312 is completed, the keysection 302 is allowed to axially move 320 further in a proximaldirection. However, without counter-rotation 318, the key section 302 islimited in distal axial movement by the distal axial stop 314. Furtherrotational movement 312 and counter-rotational movement 318 may belimited as described above by the extents of the channel interfacingwith the extents of the key section 302.

In one embodiment, counter-rotation 318 is allowed, thereby allowing thedisassembly of elements (e.g., the distal anchor element, the rigidelement). In another embodiment, counter-rotation 318 is limited, forexample, through traps, locks, spring-loaded features. Any known mannerpreventing counter-rotation 318 of the key section 302 may be employed.

FIG. 4 shows an embodiment of an intramedullary medical device 400 withan alternative embodiment of a rigid element 402. The rigid elementshown includes openings 404 which allow bodily fluids to pass throughthe rigid element 402 to/from the contracting element (not shown in thisview). The transit of bodily fluids may result in beneficial infectioncontrol and acceptance of the intramedullary medical device while insidethe patient. The openings 404 are shown placed in a manner whichmaintains the rigid element's 402 resistance to torsion and bending,while still allowing fluids to transit through the rigid element 402.Other configurations of openings 404 may be used as alternatives to theillustrated configuration of openings 404.

The rigid element 402 includes a flute 406 which extends through theproximal anchor element 410 to the proximal end 408 of the rigidelement. The flute 406 may be situated to guide body fluids along theflute to/from different parts of the bone bore (in which theintramedullary nail is placed within a patient) and to/from the openings404. As further described above, the flute 404 may also be designed tostrengthen the rigid element 402 through buttressing and/or variationsdue to the flute 406 in the cross-section of the rigid element.

FIG. 5 shows an alternative embodiment of a contracting element 500 foran intramedullary medical device. In the embodiment shown, thecontracting element is a tube (e.g., composed of SMA) which has voids502 (e.g., slots, openings) disposed in a circumferentially-spacedarrangement. The voids 502 may be designed to control the effectivevolume of the material of the contracting element, thereby controllingthe compliance characteristics of the contracting element 500. Forexample, the voids 502 can reduce the effective cross-sectional area ofthe SMA which makes up the contracting element 500. Alternativeembodiments of contracting elements are described further herein whichalso allow control (e.g., increasing, reducing) cross-sectional area ofthe SMA, for example, the use of a plurality of rods.

FIG. 6 shows an embodiment of an intramedullary medical device 600 witha distal anchor element 602 which extends outside the rigid element. Thedistal anchor element 602 may include an interface element 604 and aperipheral element 606. In the embodiment shown, the interface element604 is partially inside a rigid element 610, and slidably interconnectswith the rigid element. Slidable interconnection with the rigid element610 is described further above with respect to further embodimentsincluding, for example, ridges and channels. Connections between theproximal anchor element 614 and the rigid element 610 are also describedabove with respect to further embodiments.

In the embodiment shown, the contracting element 612 extends within theinterface element 604 and attaches with the distal anchor element 602near the connection between the interface element and the peripheralelement 606. In another embodiment, the contracting element 612 attacheswith the interface element 604 at or near the proximal end of theinterface element.

The peripheral element 606 may flare or otherwise have a larger diameterthan the interface element 604. In the embodiment shown, the interfaceelement 604 has a smaller diameter than the peripheral element 606 tomaintain a consistent outer diameter of the entire intramedullarymedical device, while allowing for the thickness of the wall of therigid element 610. Therefore, in the embodiment shown, the rigid element610 extends only part of the way between the proximal anchor element 614to the distal anchor site.

The peripheral element 606 contains anchor interface holes, as describedfurther above. The anchor interface holes of the distal anchor element602 allow the distal anchor element to be fixed to the patient'scalcaneus at the distal anchor site. In the embodiment shown theperipheral element 606 is approximately situated at the distal anchorsite.

In the embodiment shown, there are guiding elements between the rigidelement 610 and the interface element 604 which limit torsional andbending movement. These guiding elements are described in further detailherein with respect to elements of distal anchor elements and rigidelements which perform similar functions, such as ridges and channels.

1. An intramedullary medical device adapted to provide contractingforces between a patient's bones, the intramedullary medical devicecomprising: (a) an encasement adapted to connect under compression atleast two bone pieces; (i) wherein a proximal end of the encasement isadapted to be fixed within a first bone piece of the at least two bonepieces; (ii) wherein a distal end of the encasement is adapted to befixed within a second bone piece of the at least two bone pieces; (b) adistal anchor element contained within the distal end of the encasement;(i) wherein the distal anchor element is adapted to be fixed to thesecond bone piece; and (c) a non-linear contracting element containedwithin the encasement; (i) wherein the non-linear contracting element isconnected between an internal surface of the encasement and an internalsurface of the distal anchor element; (ii) wherein the non-linearcontracting element is adapted to provide compression between the atleast two bone pieces through moving the distal anchor element withinthe encasement toward the proximal end of the encasement; (iii) whereinthe non-linear contracting element comprises a shape memory alloyadapted to exhibit a pseudo-elastic stress-strain response over apseudo-elastic range of expansive strain imparted to the non-linearcontracting element based on the distal anchor element being moveddistally with respect to the proximal anchor element; (iv) wherein thenon-linear contracting element is further adapted to hold the at leasttwo bone pieces in direct contact and under compression through theencasement, the distal anchor element, and the proximal anchor element,while the non-linear contracting element provides compressive forces viathe pseudo-elastic stress-strain response of the shape memory alloy as aresult of the expansive strain imparted on the shape memory alloy. 2.The intramedullary medical device of claim 1, wherein the encasementinclude a unitary exterior surface adapted for interfacing with the atleast two bone pieces.
 3. The intramedullary medical device of claim 1,(i) wherein the distal anchor element has a distal bone screw interfacedisposed within the encasement; (ii) wherein the encasement includes anaccess port positioned outside the distal bone screw interface; (iii)wherein the access port is adapted such that the distal bone screwinterface may be accessed in configurations of the non-linearcontracting element having the pseudo-elastic range of expansive strain;and (iv) wherein the non-linear contracting element is adapted such thatthe pseudo-elastic range of expansive strain provides through theencasement compressive forces between the at least two bone pieces. 4.The intramedullary medical device of claim 3, wherein the access port isfurther adapted to restrict access to the distal bone screw forconfigurations of the non-linear contracting element that providethrough the encasement expansive forces between the at least two bonepieces.
 5. The intramedullary medical device of claim 1, wherein thedistal anchor element is adapted to allow direct access to thenon-linear contracting element through the distal anchor element.